Rule 5 draft

The Rule 5 draft is a Major League Baseball player draft that occurs each year in December, at the annual Winter Meeting of general managers. The Rule 5 draft aims to prevent teams from stockpiling too many young players on their minor league affiliate teams when other teams would be willing to have them play in the majors. The Rule 5 draft is named for its place in Major League Rules. (It is sometimes mistakenly referenced with a Roman numeral.) The June Rule 4 draft, known as simply "the draft", "amateur draft", or "first year player draft", is a distinctly different process in which teams select high school and college players.

Prior to its current incarnation, from 1959 until the creation of the Rule 4 draft in 1965, top first-year prospects of teams were exposed to the Rule 5 draft. This was in some ways an outgrowth of the Bonus Rule that existed from 1946 to 1950 and from 1953 to 1957.

As in the amateur draft, the selection order of the teams is based on each team's win-loss record from the prior regular season, each round starting with the team with the worst record and proceeding in order to the team with the best record. Any player selected under Rule 5 is immediately added to his new team's 40-man roster; thus, teams who do not have an available roster spot may not participate in the Rule 5 draft. Players who are not currently on their team's 40-man roster are eligible to be selected in the Rule 5 draft, but only after a standard exemption period has elapsed. See Selection eligibility below.

If chosen in the Rule 5 draft, a player must be kept on the selecting team's 25-man major league roster for the entire season after the draft—he may not be optioned or designated to the minors. The selecting team may, at any time, waive the Rule 5 draftee. If a Rule 5 draftee clears waivers by not signing with a new MLB team, he must be offered back to the original team, effectively canceling the Rule 5 draft choice. Once a Rule 5 draftee spends an entire season on his new team's 25-man roster, his status reverts to normal and he may be optioned or designated for assignment.

...
Wikipedia